text:
Do propeller planes give contrails like jets do?
Last weekend, while watching the second world war film "Fury" with a friend, there was a scene near the end with a large set of bombers flying overhead, leaving large contrails. That seemed wrong to me, as I'd thought that contrails were a jet thing, but the friend I was watching it with thought it was right. (Was a lovely shot though, accurate or not!)
That has me wondering - do propeller planes generate contrails like jets do? And if so, is it only turborops that would, or is it something all propeller planes can create?

Yes they can. That image from wikipedia showing B-17s is more descriptive

Contrails originate from condensation of vapor coming from internal combustion engines, regardless of being turboprop, turbojet/turbofan or cylinder engine. Vapor is a byproduct of the hydrocarbon (fuel) being combusted with the atmospheric oxygen.

Just an addition, some contrails form due to lower pressure areas at certain angles/speeds of flight. Wingtip contrails and propeller contrails are good examples.

@Alexus Yes but in most cases it comes from combustion and it's more prominent and visible from long distances as you can see from the image in my answer. Also OP asked _ do propeller planes generate contrails like jets do?_ The effect you describe can happen on a jet engine but will never be visible as a trail behind it.

@Alexus, the low pressure "vapes" you describe is a very fleeting phenomenon compared to the persistence of exhaust contrails.

@MichaelHall Sure, but they are a real thing.

"Contrails" is a contraction of "condensation trails" (as opposed to "chemtrails" - a contraction of "chemical trails" [think "trails" left by an agricultural crop duster aircraft as it makes an application pass over a field]).
All other things excluded, condensation trails can occur when specific conditions in the atmosphere occur - certain levels of moisture (visible or not), the mass of air that contains that moisture moving from one area of pressure to a lower area of pressure, and that mass of air moving at a velocity that is commensurate with producing a change-of-state of the moisture in that mass from a non-visible state to a visible state.
When non-visible-moisture-laden air moved rapidly from a high-pressure area to a low-pressure area a rapid change of air temperature takes place due to the expansion of the space between air molecules.  In cool air water molecules clump closer together than they are in warm air, sometimes to the point that the moisture actually becomes visible.
All other things being equal, the faster the velocity of the moving air mass from the high-pressure area to the low-pressure area the more likely visible moisture will appear.  That is one reason you can see visible moisture coming off the propellers of the C-130's in the photos which have been submitted to this website.  The propeller tips are moving at nearly the speed of sound, their angle-of-attack relative to the air in front of them is such that they move a huge amount of air past them at a very high speed.  The amount of moisture in the air is such that when it undergoes a compression/de-compression cycle as it passes the tip of the propeller blade is is quickly compressed then de-compressed and then is appears as vapor behind the propeller.
As one contributor has already explained "contrails" can also come from exhaust systems of piston-driven aircraft.  Those "contrails" come from basically the same events - i.e., compression and then rapid de-compression of moisurized air.  The most-often seen photographs/movies which show this effect are usually of B-29 aircraft.  Most piston engines cannot operate at altitudes higher than about 20,000 feet because the air is too "thin" to sustain combustion in their cylinders.  B-29's, like many other late-WW 2 aircraft - were equipped with "superchargers" - in effect very powerful "fans" which pulled in atmospheric air, compressed it, then sent it into the fuel/air intakes, thus providing sufficient oxygen to sustain the fuel burn in the cylinders.  That "compressed" air naturally contained moisture - some of which did not completely get enough heat/pressure in the cylinders to be destroyed.  It subsequently re-formed as condensation after it left the exhaust stack - only with a vengeance because of the extreme temperature shock it experienced going from the high temperature/pressure of the combustion cylinder to the sub-zero temperatures of altitude at the high altitude these bombers were flying at.

Welcome to Aviation.SE!

This is excellent, apart from the last few sentences. Internal combustion engines are not an environment where water breaks down chemically. The contrails from engines are not because water enters the engine and "is not destroyed"; they're because fuel enters the engine and is burnt to produce water, carbon dioxide, and small amounts of other substances. Roughly speaking, every kilo of fuel that's burnt produces a little over a kilo of water and three kilos of carbon dioxide.

"chemtrails" is more usually reserved for the conspiracy theorists who claim that contrails are actually "secret mind control chemicals" sprayed from aircraft by ...

RE: response by David Richerby

Perhaps a bit misleading to to refer to WW2 contrails only being possible in 'late war aircraft'. Think of the photographs of the Battle Of Britain showing trails all over the sky as spitfires & 109s fought over Britain in 1940.

I'm all for reclaiming the term Chemtrails

Yes they do. They can however have different types of contrails. Some come from propeller blade tips, while others are from engines themselves.
Here are a few examples:
Video of A Hercules C-130:
C-130 Contrails

From: Contrail Science

From: Airliners.net

These are only short, because no water gets added in them, so they evaporate again as the vortex slows. Note that they basically don't extend aft of the wing on your second picture. Why they seem to extend further in the first picture is that as the pressure returns to normal, the exhaust plumes add the water to keep it condensed.

These are not what are usually meant by the word "contrails", i.e., long-lasting trails of condensation behind the aircraft.

You are correct, @DavidRicherby, but they make for some incredible pictures!

@FreeMan You area also correct. :-)

@DavidRicherby Technically, these also are contrails, as they are condensation, and they are trails behind the wing(which we can assume the propeller to be a wing of it's own type) :)

@Alexus That doesn't contradict my statement that they are not what are usually meant by the word "contrails".

@DavidRicherby Fair enough :)

I wonder if this can be reproduced in a small-scale experiment with a household fan in cold weather. And if there is a video out there showing it.

Probably a fan large enough. You need some fan speed that will generate enough negative pressure.

Almost all fighter and bomber aircraft had supercharges from the very start of WW2. So all of them would produce contrails at high altitude. 30,000 feet was no problem to most of these early piston engines aeroplanes.
The comment about the B-29 being one of the few that produced contrails was very misleading.


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Can the ILS glideslope be intercepted from above?
Is it recommended to intercept the ILS glideslope from above? Are there any special considerations like unstable approaches or GPWS warnings?

Assuming that you're asking about intercepting the glideslope from above rather than the localizer, the answer is that it is definitely NOT recommended.
There are at least two significant problems if you do this. First, is the fact that at high vertical angles there can be false glideslopes. Looking at the diagram below, if you're coming at the glideslope from above and the needle starts to move, are you intercepting the real 3° glideslope or the false 9 ° glideslope?

The diagram above is the best I could find, but it has one error. It shows a "Flag no signal" at 6°. Actually what you'll have there is another false glideslope, but one which has reverse sensing.
You can, of course, do a reasonability check if you know your distance from the runway, or you could compare your vertical speed if following it with what you know your vertical speed should be, or on the 6° line a command to go up when you know you should go down would be a hint, but it's unwise in my opinion to put yourself in a situation that increases your work load that much during an instrument approach.
You asked about GPWS warnings. None of the aircraft I flew that had GPWS would sound a GPWS warning just from receiving a false glide slope. However, all of them would have sounded the GPWS if you followed a false glideslope and were close to the ground because doing so would give you an excessive sink rate.
A second problem with approaching from above is that the vertical angular width of the glideslope as measured from needle full down to needle full up is very small. As I remember only about one degree. Whatever it is, though, the point is that if you descend into the glideslope with a vertical speed of only a few hundred feet per minute greater than the glideslope itself is descending, the needle is going to go from full down to full up very quickly. Without working it out, I'd guess that if you're inside the outer marker you might well miss its movement if your instrument scan had you away from the needle more than a second.
In the late 1970s a friend of mine was flying a charter in a Cessna 310. He unwisely decided to intercept the glideslope at Salem, Oregon from above. The aircraft hit the ground in the vicinity of the outer marker, killing all aboard. During the accident investigation, the NTSB found from radar data that he had a descent rate greater than 1,000 fpm going through the glideslope.

What causes false gbdislopes?

@TomMcW I don't know. However, your comment induced me to include a false glideslope diagram in my answer. Let me encourage you to ask that as a proper question. I'm sure there are others here who can answer that if it hasn't already been answered, in which case when you start to ask the question the site may show you that the question has already been asked and answered.

That question is asked here but doesnt get a real in-depth answer as to the causes. I can't find a good explanation as to the causes on the net anywhere. BTW sorry to hear about your friend.

Approaching the glideslope from above was also cited as a contributing factor to the crash of Turkish Airlines Flight 1951 in Amsterdam.

@TomMcW The only thing I found was, "This is because the transmitter on the ground generates a mirror 150Hz sidelobe just above the 90Hz main lobe." I haven't the slightest idea what that means.

@TomMcW, the false glideslope is caused by the way interference is used to generate the signal. Since constructive interference occurs when the distance to the aerials is any integral multiple of the wave-length, the signal designed to interfere at the intended angles also interfere the same way at multiples of those angles.

@JanHudec In essence, yes, although a separate question would be better for complete answers.  In fact, I believe that the diagram Terry provided is incomplete.  It is possible to capture at 6 degrees but the sense is reversed, i.e. if you are low, you will get a pitch down command - oops. Equally deadly is the 9 degree one which will have you hurtling towards the threshold at 3 times the descent rate - but that's why smart pilots don't just chase the needle ;) (ex avionics engineer here)

@JanHudec BTW, the 12 and 15 degree (and other lobes) are so weak that they will be rejected by the filters on the receiver.

@Simon, yes, I wasn't exactly sure how the diagram should exactly look, but I realized this one is not fully correct.

@JanHudec I agree, the diagram is incorrect in not showing the reversed false glideslope at 6°. It also shows the angles as wildly exaggerated, but that's okay I think. I'll look for a better diagram or at least put a note on the existing diagram. And I think a separate question is a good idea.

All antennas have side lobes, and localizers have them as well. The design prevents a side lobe below the main beam (at least any significant one). Arcing DME approaches do not use the localizer beam alone for the turn to final. Pilots must fly the published approach procedure to correctly establish the aircraft on the beams. Electronically scanned antennas can significantly reduce side lobes.

If you mean the glide slope (vertical guidance) rather than the localiser (horizontal guidance), then you should capture from below.
The reason is because if you capture from above, you must be descending and there is a risk, particularly if you capture the glide slope late, that you will descend below it.  This is a Bad Thing™.
The approach, if followed, or ATC vectoring will bring you onto the localizer (horizontal guidance) at an angle, typically 30 degrees or so. You capture the localizer signal first, then then fly at a fixed altitude until you capture the glide slope, at which point you start descending.

Another important factor in intercepting a glideslope from above is speed. This is particularly relevant for faster jet aircraft. Jets will typically be able to descend or slow down, but will have a hard time doing both at the same time.
If the jet needs to intercept the glideslope from above before slowing down enough to extend flaps and landing gear, the spoilers may not provide enough drag to slow down much. If the aircraft does manage to intercept the correct glideslope, the aircraft may be traveling faster than usual while still descending on the glideslope. Going too fast at this point can qualify as an unstable approach, and increases the risk of a late touchdown and a runway overrun due to the excess speed.
If the aircraft is intercepting from above with flaps and gear extended, and engines at idle, the pilots should take care that when the glideslope is intercepted or a go-around is performed, the aircraft is not pitched up too far before the engines are allowed to develop sufficient power to overcome the high drag of this configuration.

There's never a reason to do this.  Proper Planning.  If too high at the FAF request vectors back around and set it up correctly.

It isn't common, but there ARE real-world situations where it is necessary to do exactly this. A bad vector from ATC, you're 12 miles out & 1 dot high, "can you make it from there?"  Yes, absolutely you can in a case like that, and getting vectors is a waste of everyone's time. It's a reasonable tool to have in your bag of tricks.

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Could a SR-71 be shot down today?
I've read that the SR-71 had no missile defenses because it could simply outrun any adversary. Is this still true? Or is it unlikely that the SR-71 could operate unimpeded in the face of modern technology?

Seems there is nothing new. Mig-25 vs SR-71. And no newcomer in the Top 2.

Umm - yes!

Well, the SR-71 is essentially a really really big flying missile when it is in the air and we have some pretty good antimissile systems so I would take a guess and say most definitely yes.

Could SR-71 be shot down today?

Theoretically yes. Modern day SAMs like the 40N6 that travel at speeds of Mach 6 can intercept an SR 71 within an operational range of 250 miles upto an altitude of 18 miles. Moreover, the hostile SAM will fire a salvo of missiles(at least 2-3). So the SR 71 will have to jam, drop decoys and out manoeuvre the incoming hostile missiles.Modern day SAM systems can be loaded with libraries of 3D shapes that can be used by the seeker to identify targets and threats.
S-400 and similar Russian/Chinese AD systems are meant to work simultaneously with ECM systems, meaning radars of enemy aircraft will be blinded  as 40N6 and 48N6 missiles zeroes in.
During the days of the cold war Soviet Air Force pilots were instructed to launch two rounds, a semi-active radar homing missile and a heat seeking missile.In this fashion the SR 71( or any other hostile aircraft) being targeted has a difficult problem as it must jam, decoy and/or out manoeuvre three or four tightly spaced inbound missiles
That being said getting in the general vicinity of an intercept is different from an actual intercept.
http://www.codeproject.com/Articles/19310/Hitting-a-Moving-Target-The-Missile-Guidance-Syste
From Jane's  Fighter Combat in the Jet Age : Page 116-117 “SR71 the Ultimate Target”

On June 3, 1986 an SR 71 was on a mission over the Barents Sea. Six
  Mig 31 Foxhounds, vastly superior to the Mig25 Foxbat,performed a
  coordinated intercept that would have subjected the SR71 to an
  all-angle AAM attack that even a combination of high-altitude
  maneuverability and ECM could not have defeated. Fortunately for the
  American jet, the interception took place over international waters,
  but the Soviets proved their point.

In fact,the SR 71 was retired once the USSR started deploying S 300 SAMs in large numbers.

I've read that SR-71 had no missile defenses because it could simply outrun any adversary. Is this still true?.

That is not true. The SR 71 did carry a very sophisticated Electronic Counter Measure System(ECM) called the Defensive System
One of the first MiG 31 Foxhound pilots, Captain Mikhail Myagkiy, who had scrambled with his MiG-31 several times to intercept the US super-fast spy plane, explains in his book how he was able to lock on a Blackbird on Jan. 31, 1986:

The scheme for intercepting the SR-71 was computed down to the last
  second, and the MiGs had to launch exactly 16 minutes after the
  initial alert. (…) They alerted us for an intercept at 11.00. They
  sounded the alarm with a shrill bell and then confirmed it with a
  loudspeaker. The appearance of an SR-71 was always accompanied by
  nervousness. Everyone began to talk in frenzied voices, to scurry
  about, and react to the situation with excessive emotion.

Missiles are faster and can sustain higher G loadings than SR-71.  Assuming that the SR 71 is flying at an altitude of  70,000 ft to 80,000 ft  it only needs to maneuver to out turn the missile, in order to  make the change in intercept geometry fall outside the missile's envelope. However, at extreme altitudes aircraft maneuvers are limited by stall buffet.
The SR-71 could fly a 7,000+ mile mission and the only segments of which are flown at sub sonic speeds  are the take-off, refueling  and landing.
http://www.sr-71.org/blackbird/
http://www.sr-71.org/blackbird/diagrams/index.htm
http://www.sr-71.org/blackbird/diagr...conprofile.htm
The thing is the Soviets knew that in order for any Soviet fighter aircraft to intercept an SR-71 it had to be ahead of its known flight path. These were the tactics the Soviets deployed to even get a glimpse of the SR-71. They always knew when an SR-71 launch would occur because the KC-135 fuel tanker would take off first because the SR-71 had to be immediately refueled once airborne.

Two of the links to sr-71.org seem to be broken.

I think that tj1000's answer is a rather romantic version.
The SR1 stopped overflying the USSR because the MIG31 Foxhound had the legs to close on the SR71 and vector it's missiles if it took off at the right moment.
Once the Soviets had locked on once and on another flight 'boxed in' the SR71 over international waters such that they had a clear kill option available, it was all over for the SR71.
The SR71 would today be toast.

Even if they cannot hit it they surely can make it too busy to properly carry on it's mission (deep in enemy teritory). Considering the costs of each fly the opposition just needs to hind the SR71 long enough retiring it will be the only option left. At the bototm line my guess is survelaince sattelites can do a better (and safer) job

As far as I know, the SR71 never overflew the USSR. That was stopped after the Gary Powers U2 shootdown. Politically, too expensive to have that happen again. The USSR did shoot down several US recon aircraft in international airspace during the cold war, all were subsonic. And the MIG31 still only has Mach 3 dash capabilities, for brief periods. Catching a single aircraft that can go Mach 3 for over an hour would take a hundred MIG31's on alert at a lot of locations, and maybe they could get it.

Certainly yes. This Wikipedia page states several missiles:

These have a greater speed than the SR-71 and a greater flight ceiling. Aso, if white house decides to turn its own enemy, then it can create a havoc with SM-3 Block 1B.

Higher speed and flight ceiling doesn't necessarily mean it can be shot down -- the SR-71 has a very good head start on missiles that are launched against it, meaning range is critical.

and then there's reaction time, detection range, etc. etc. Your cone for a successful intercept at those ranges and speeds is extremely small, same problem you have with intercepting ballistic missiles and even worse, the target can turn out of the cone...

Agreed with cpast and jwenting. The missile has to be so fast that it can close the distance between it and the SR-71 before it runs out of fuel.

On paper, a 1980's vintage SR71 with 1980's countermeasures and support, would probably be vulnerable to the latest SAM's.
However... if a nation were to field a hypersonic aircraft with the SR71's capabilities today, they would outfit it with the latest stealth technology, support, and countermeasures. And those haven't remained static since the 1980's, either. Stealth means harder to detect (not impossible) which means shorter lead time... a real issue on a target moving that fast. Countermeasures mean more convincing decoys. Better support could be a few F35's down low, ready to pounce on any radars that go active. Maybe that's why a contemporary SR71 would be flying that mission - to get those sites to give their location away by providing a tempting target.
I suppose one could optimize a SAM to hit a hypersonic aircraft with the latest tech, but as no hypersonic military aircraft exist today, there is no reason to go to all of the expense. SAM's have been optimized to hit ICBM's, but ICBM's don't carry countermeasures or maneuver radically. They follow a fast but predictable ballistic arc.
If we take as a measure the last major threat to the SR71, it would probably be 1982, and the Libyan air strike. A SR71 flew post strike recon, when the Libyans were on full alert and mad as a nest of hornets. The Libyans threw everything they had at the SR71 and couldn't hit it, though the pilot did report 'Mach numbers he's never seen before' in completing the mission, in addition to using every ECM they had at their disposal. That was the latest US tech versus the latest Soviet tech in that era. Prior to that, a SR71 flew recon during the Yom Kippur war, and was shot at by both the Egyptians and the latest Soviet missiles, and the Israelis and the latest western missiles... as that SR71's flight hadn't been announced to them. Neither side could hit it with the tech they had.
The fact is, when the SR71 was operational, the best Soviet tech of that time couldn't stop it. Russian and Chinese missile tech has increased considerably since that time, but so have countermeasure and support tech.
Finally, the SR71 wasn't retired due to vulnerability to newer SAM systems. It was retired because the cost of operation was very high, while satellite surveillance had increased in capability at a much lower operational cost. If the USSR contributed to the demise of the SR71, it did so by imploding and reducing a major strategic threat, and the need to spend a fortune on a hypersonic aircraft to assist in balancing that threat.

ICBMs do carry countermeasures...

You have it backwards. Hypersonic missiles (anti-ship, icbm, Sam ) will be operational and widely deployed before hypersonic aircraft.

They still fly the U2, so aerial surveillance still plays an important role.

I don't think this makes sense. The whole point of the SR71 was to fly fast and high over places where you couldn't fly low, but you're proposing to accompany your "SR71-neo" by slower, lower planes, negating both of its advantages. You also suggest that the SR71 could cause defensive radars to give away their position to the lower-flying planes -- but the lower-flying planes could do that on their own, so what is the SR71 contributing?

Sadly, the SR-71 (my favorite) would be an easy target these days.
The IR heat signature from the engines and nacelles make it readily visible.
In addition to the amazing information regarding missles from the posters above,
there are distant horizon laser tracking systems and chemical lasers which can take down any aircraft (the USAF ABL system).  There are also some newer ground-based fiber lasers which can do damage to composites on both missles and aircraft at long range.

Actually, no currently operational laser system can damage an SR-71 at any relevant range/altitude. Even with perfect tracking, detection at infinite range, no clouds, and low humidity, the spot size would be too large and the intensity too low to cause any significant damage (cameras might be blinded though). Besides, unless the SR-71 flew almost directly overhead, attenuation alone would absorb most of the beam power. The [canceled] YAL only had to contend with thin-skinned, low structural-margin rockets. Even with a 1+ MW COIL, it would probably struggle against Ti designed for 300°C.

The Lockheed A12, the predecessor to the SR71, operated by the CIA in photo reconnaissance missions over North Vietnam could sustain a maximum altitude of 95,000 ft and Mach 3.5. None were shot down, but one landed after a mission with a piece of shrapnel from an SA2 missile embedded in the underside of it's wing. In the 1950s, Surface to Air Missile systems were in their infancy, notwithstanding the development work done in Germany and Switzerland during the Second World War, and the prevailing view was still that aircraft could avoid interception by flying very high and fast, or simply very high as in the case of the Lockheed U2. This view of things was attractive at a time when rapid advances in engine and airframe technology were making possible equally rapid increases in aircraft speed and altitude capability.

The Blackbird could have shot down by the Swedes, as early as 1977.
SR-71s based in the UK would either take a route north along the Norwegian coast or east across the Baltic Sea between Sweden and Denmark.
This article is in Norwegian, so I have used Google Translate for the following two passages. The Swedish Air Force claims to have tracked and achieved "firing team contacts" on 51 occasions:
"I løpet av de 322 toktene SR-71 gjennomførte over Østersjøen mellom 1977 og 1988, gjennomførte de svenske Viggen-pilotene 51 skuddhold-kontakter ifølge Flygvapnets egne rapporter."
"During the 322 journeys SR-71 carried out over the Baltic Sea between 1977 and 1988, the Swedish Viggen pilots carried out 51 firing team contacts according to the Air Force's own reports."
=============
"Boken "Lockheed Blackbird: Beyond the Secret Missions", bekrefter i ettertid fra amerikansk hold at JA37 var det eneste flyet som klarte å komme på skuddhold for missiler. Om de hadde truffet dersom SR-71 hadde gjort unnvikende manøvre er et åpent spørsmål."
"The book Lockheed Blackbird: Beyond the Secret Missions confirms in retrospect from the US team that the JA37 was the only aircraft that managed to come on missile firing teams. If they had hit if the SR-71 had made elusive maneuvers an open question."
=============
A third passage explains that this was possible because the Swedes had better integration between Ground Control and their aircraft than the Russians:
"I motsetning til de russiske jagerflyene, var JA37 tidlig ute med en avansert taktisk computer til beregning av kurs og måldata. I tillegg kunne den ta imot informasjon på datalink direkte fra stridsledelsen på bakken. Flyradaren hadde en rekkevidde på 60 km mens det taktiske displayet rakk ut til 80 km."
"Unlike the Russian fighter aircraft, JA37 was early with an advanced tactical computer for calculating course and target data. In addition, it could receive information on data links directly from the battle leadership on the ground. The aircraft radar had a range of 60 km while the tactical display reached 80 km."

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Why does the DC 4 airplane not tip backwards?
The wheelbase of the DC-4 is very short relative to the fuselage

The 4 engines weigh a combined 7,000 pounds. It would seem like even with the weight of the engines, it's not enough to prevent the plane from tipping backwards. If a really fat passenger sat in the back of the pane, it would tip.

I don't agree it is a duplicate. The question here is specifically about the DC-4, while the linked question addresses modern airliners in general.

I agree that a more specific answer would be good here. Also related: Which Aircraft Models need a Tripod Jack for tail support at airport?

@kevin I should have referenced it as "related", not really a duplicate. I was searching for the question with the close dialog and hit the wrong button.

Related: Can passenger distribution cause a parked airliner to tip?

What's your question? Clearly the centre of gravity is in front of the rear gear because the plane doesn't tip. Your entire question is just a statement that something that we can empirically see to be false must be true. Obviously, this statement is incorrect.

Related: http://aviation.stackexchange.com/questions/16447/why-is-the-main-landing-gear-of-the-cessna-caravan-placed-so-far-forward

Although the landing gear are pretty far forward on the DC-4 the weight of the engines places the center of gravity in front of the main landing gear. It's unlikely that a passenger would be heavy enough to tip it, but freight certainly can. When loading freight tipping is prevented with the use of a tail stand, as can be seen in the photos below.

Image source

Image source

Are there any CoG diagrams for the DC-4? Just wondering.

What do they use to support the tail when the DC-4 is taxiing\taking off?

@Sean The tail stand is only needed during loading and unloading. Once all cargo is in place the center of gravity will be forward of the rear landing gear, so it wont tip. But think about the loading process. The door is in the front, so you have to load the rear cargo first. It will be tail-heavy until you load the cargo in the front, then it will be balanced. If you want to see what happens if you're not careful just look closely at my icon photo.

@TomMcW: Ahh, I see now.  Gottit.

@Sean There are a couple of other ways to deal with the problem. The tail of the DC-10 is too high to make a tail stand practical, so it uses a tether to fix the front gear to an anchor point on the ground.


text:
Why do we use the length of the canard as 1/3 of the wing?
Twisted canard is our project, so we are designing and analysing the model. We compare the twisted canard with an untwisted canard to find the best among them.
The problem is we designed a canard of 1/3 length of the main wing(delta wing) as many fighter aircrafts used. So now the question is why the length should be 1/3 of the main wing?

Are you asking why a canard wing is usually one third the length of the main wing? Can you give an example of an aircraft you're using as a source?

The ratio depends on the design performance of the aircraft. Is your design supersonic (say M1.5–2)? If not, you probably shouldn't be basing it off fighter jets (the delta wings also only makes sense on supersonic designs).

I think its due to increasing the lift of the wing most optimal.
The canard will form a vortex that sweeps over the main wing and will increase lift. On the NASA technical report server there is several documents that cover canards:
NASA document
The 1/3 is probably designed so that the vortex passes over the wing in the place that will result in the most lift, while not having the canards be too large due to drag.


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How is the TCAS negotation information encoded?
As I understand, according to this FAA booklet on TCAS, transponders will engage in cycles of interrogations and responses to establish if a collision may occur.
The next step is for both TCAS systems to negotiate respective resolution advisories. My question is how the negotiation takes place in the TCAS system internally, and in particular what communication protocol takes place for the negotiation.
To elaborate, as an example, a browser can make an HTTP request to a server (like this site) which follows protocols as to what information should be provided and how it should be encoded.
Along the same vein, how is information encoded for TCAS systems to negotiate? Since I don't know of any open source TCAS used in commercial aviation, I can't inspect code.

Related

Also related

@DeltaLima Just because it is about TCAS does not make it related; my question is about how data is encoded for a TCAS negotation, and I do not find that question pertinent. It details how the algorithm works, rather than what is actually being sent for negotiations.

@user1997744 If you look at the accepted answer to that question, under the header RA coordination you will find a short description of what is being sent.

@DeltaLima yes but it’s quite obvious its intent is being sent - what else could it be? It has to say what action it wants to take one way or another. It’s the encoding that is pertinent.

TCAS air-to-air messages use the same format and frequencies as "Mode S" surveillance radar. They're defined (in great and overwhelming standardesque detail) by a document called "ICAO Annex 10".
ICAO wants serious money for a copy of it -- and of course if you're producing actual avionics nothing less than the official source will do -- but for satisfying idle curiosity it's often possible to find a draft or bootleg by persistent googling.
The Mode S message format is also used for ADS-B broadcasts, so equipment to receive and decode them is relatively easily available to hobbyists. The lower layers of open-source ADS-B software will also be generally relevant.

So ICAO Annex 10 would be what would be used to encode say, “I want to descend while you climb”, so to speak in layman’s terms? Oh and which volume of Annex 10 is relevant here?

@user1997744: If I remember correctly, the algorithm is constructed such that in most cases the two TCASes would produce complementary RAs without waiting for such an explicit negotiation, but instead comparing each other's reported altitudes and airframe addresses. But in order to get all the corner cases there are also explicit negotiations of the kind you describe, and somewhere in Annex 10 their exact format will be defined. I don't remember whether Annex 10 also contains a high-level description of the algorithm or you need to go elsewhere for that.

Okay, I’ll try to get a copy and grep for some keywords to try to find that part. Thanks.

@user199744 Take a look at RTCA DO-185B, the Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System II (TCAS II) for all the details of TCAS II algorithms and message formats. It is more detailed than annex 10 (which repeats part of DO-185B).


text:
Streamwise Pressure Gradient on Curved Walls
Could someone help me to understand how can a wall generate differents streamwise pressure gradients just by its curvature? I can understand how it works in cross-flow (perpendicular to the streamline) but i don't have the intuition neither the math approach to the streamwise direction.

Do you mean a flat wall/plate or a curved one like the one in the picture? You don't understand the "decreasing" and "increasing pressure" part of the story, correct? But you do understand the variation in y, right?

A curved one. Correct, i can't relate how there is variation in the pressure gradient streamwise oriented due to curvature, what leads to acceleration / deccelaration and separation. Yes, if i understood right it happens in y due to the resultant centrifugal force, so pressure "outside" a curved streamline is higher than the pressure applied from the "inner side".

Regarding the pressure and velocity variation along x I'd suggest you to read this very good answer, especially the part "Flow over the upper side of the wing". Regarding the variation in y, this is going to help. And no, it has nothing to do with centrifugal forces

About 'y' direction: i wasnt talking about velocity gradient in the normal direction and close to the wall, but how a pressure field vary in the normal direction when the streamline is curved, like in this paper: /https://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf
With this we can realize how the normal gradient of pressure is given for a flow over an curved shape.

'x' direction: I don't know if i understood it right but, to maintain the flow aligned with the curved shape, the pressure acting on the flow direction must adjust the velocity to not create 'vacuums'?

Wow, that's a really good paper, I'm going to note that down . And now I understand what you meant with pressure variation in y and centrifugal forces. Yes, you're understanding is correct, included the vacuum story (but not in the boundary layer as depicted in your picture where friction among the different layers of fluid makes the physics a bit more complicated). The math in the appendix should also be easy to follow and gives a good background to the text in the paper.


text:
What is the fuel burn of a 787-9 on a ferry flight?
I can't find any literature on 787-9 fuel burn for ferry flights, meaning no cargo or passengers, just crew.
An 787-9 burns about 5400 litres when on normal commercial service.
How does it change when on a ferry flight?

The amount of missing information necessary to solve this is far greater than the amount of information provided, making the question entirely unanswerable in its present state.  If you have "upcoming ops" involving a real aircraft, then whoever is operating it can provide much more specific information about your specific operation than a message board ever could (especially given the utter lack of necessary details here). VTC

You can figure out the fuel burn using the Breguet equation (similar to how I did in this answer):
$$ \frac{\text{TOW}}{\text{LW}} = \exp \left( \frac{R \cdot g \cdot b_f}{v \cdot L/D} \right) = \exp(f \cdot R) $$
Here, TOW is the takeoff weight, LW is the landing weight and $R$ is the flight range. I grouped all the constants together into a factor $f$. The TOW consists of the OEW (Operating Empty Weight, 128,850 kg) plus the total fuel. The LW is then the OEW plus the fuel reserves (let's say 5000 kg).
To determine the unknown factor $f$, we look at the Payload-Range-Diagram, which can be found in the Boeing Airplane Characteristics for Airport Planning document:

The last line on the right limits the range based on maximum fuel capacity. On a ferry flight, the payload is zero, so we can read off the maximum range at full fuel (around 9400 NM). Note that this is larger than the ranges you would typically find (e.g. 7645 NM on Wikipedia) because those ranges includes typical payload.
Now we can plug in the number into our equation and solve for $f$:
$$ f = \frac{1}{R} \ln \left( \frac{\text{OEW} + \text{Fuel}}{\text{OEW} + \text{Reserve}} \right) \approx \frac{1}{9400 \, \text{NM}} \ln \left( \frac{230 \, 294 \, \text{kg}}{133 \, 850 \, \text{kg}} \right) \approx 5.773 \times 10^{-5} \, \text{NM}^{-1} $$
With the known $f$, we can now plot the fuel burn on a ferry flight as a function of range:
$$ \text{Fuel Burn} = \text{Fuel} - \text{Reserve} = \left( \exp(f \cdot R) - 1 \right) \cdot (\text{OEW} + \text{Reserve}) $$

This would be the total fuel burn on the ferry flight. To get the fuel rate, we divide by the flight time (assuming a typical cruise speed of 488 kt):

The 787-9 therefore burns between ~3.8 and ~5.0 t of fuel per hour on a ferry flight.


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Do landing gear doors need repairs after a gear gravity deploy landing?
I know that landing gears can be deployed even without hydraulic power by letting them go down by gravity, as seen on this video:

I was wondering: do the landing doors need replacement after the landing? It looks like the get damaged a lot, especially on hard touch downs.

No, they do not get damaged.
The Main Landing Gear (MLG) or Gear Bay doors can and are regularly opened on ground for maintenance purposes. Even with fully compressed oleos the MLG doors are still off the runway surface. Have a look at this Air Canada A320. You can clearly see the doors being well above the bottom of the MLG.
If you came to your conclusion by watching the white puff of smoke coming from the area of the open MLG doors - that's actually the burning rubber from the nose gear touching down.

It really depends on the aircraft type. There are some aircraft that will drag the doors on the ground if there is no hydraulic pressure. (BAC 1-11, DC-9, Fokker F-28, etc)

uhm, looks like there is little clearance between the landing doors and the ground, a wheel radius worth of space. At least on a A320 -> https://www.youtube.com/watch?v=g97WyYvKLfQ

The problem with non-hydraulic deployment is that many planes will not positively lock the gear down, so it becomes a guessing game whether the gear will collapse when the runway is reached.  In a TV series called Ice Pilots NWT, flying out of Yellowknife, North West Territories in Canada, they had a Lockheed L188 Electra where the gear got stuck on the doors on one side. Pilot tried landing on the good gear twice hoping to shake it loose with no luck. Ended up landing on one main and nose gear, and tore the low wing props all up as it came down the runway, finally stopping with no injuries.

@CrossRoads, the L-188 is a design from the mid-1950s.  Design standards have changed a bit since then.

My initial reaction was that there was just tire smoke. However, it appears that there may be some sparks shortly after MLG touchdown at about 0:27/0:28. Upon further watching, that may be smoke from the NLG, though.


text:
Can the Harrier jet really take off both conventionally and vertically?

What I am able to understand from the above video, harrier jets have both capabilities:
1)To take off horizontally like other conventional aircraft like the F16 (because it has also tires)
2)To take off vertically like helicopter
Is my understanding correct?

What is your confusion?  Why do you think that might not be accurate?  What research have you done to try to figure it out?

I just want to confirm, is it ocrrect that i can take off horizontally? As in most videos, i have seen it takes off vertically

The video shows one taking off horizontally.

Have you actually had a look at the video you posted? Both kind of take off are shown there

The vertical take off was more a "parlor trick", only possible with very light loading. For the most part a short or conventional takeoff was used.

The Harrier Jump Jet, though capable of taking off vertically, can only do so at less than its maximum loaded weight. In most cases a short take off is needed to lift the required amount of fuel and weapons needed for a training sortie/mission, using forward speed to supplement the jet lift with aerodynamic lift.
Source

Yes, but vertical landings were usual...

@xxavier indeed, when you've burnt off plenty of fuel and dropped all your ordnance, then it was absolutely doable.

I was at an airshow once where they said they couldn’t demonstrate pure vertical takeoffs or landings because the runway surface was tarmac and the jet blast would destroy it.

@Jim I have similar memories at farnbourough airshow in the late 90s or early 00s

V22 Osprey has the same issue

@sophit even pure helicopters can have close to the same issue, I remember a Vietnam War Huey pilot's recounting of being in that situation in his memoir (Chickenhawk). Had to basically slide on the skids for a while building up speed until they could get in the air

@llama: I meant that when the osprey is in helicopter configuration the jet blast of the turboprop can destroy the tarmac where it lands/takes off. If I remember well it cannot land/take off from all aircraft carriers just for that reason


text:
What happens without trimming a statically stable aircraft?
if I have a statically stable aircraft, it will always return to its neutral position. What happens now if I do not trim my aircraft? Will it move/osicallate around its neutral position and be statically stable but dynamically neutral?

An aircraft is always trimmed for some airspeed. Manual control input allows you to force a different airspeed from what the plane is trimmed for, but if you let go of the controls, the plane will return to its trimmed speed. Depending on power setting, that will result in either a climb (to slow down) or descent (to speed up). This is why you can climb or descend by adjusting power without ever touching the yoke/stick.
(I assume that by “trim” you’re talking about elevator trim; rudder or aileron trim would have corresponding effects in their own axes.)

Trimming or not trimming your aircraft should not change its inherent stability. It will, however, change the airspeed, vertical speed, and attitude of the aircraft based on power settings. Control input would have to be applied in order to maintain these parameters. The more out of trim the aircraft is, the more force/pressure will need to be applied to the controls.


text:
Did we get closer to another plane than we were supposed to, or was the pilot just protecting our delicate sensibilities?
Yesterday, April 28, 2019, I was flying on Alaska flight 557 from LAX to PDX.
About an hour into the flight (near the California/Oregon border) I was looking out the window and saw a jet travelling the opposite direction from us.  It was travelling exactly the opposite direction of us and I saw it when it was ~45-60 degrees forward of us.
Within a couple of seconds of my seeing it, we banked sharply to the right.  It was not a violent turn; An answer to this question describes true evasive maneuvers as, "those of you not strapped down would have been hurled to the ceiling or slammed to the floor and your stomach would be heaving in a different direction from the rest of you." - and we didn't have anything like that.  I'm not sure I would have even noticed the turn had I not been looking out the window.
However, it was a pretty sharp mid-flight maneuver, and because I was over the wing, I didn't get to see the other airplane at closest approach because the wing blocked my view.  I would estimate that it was about 3-5000 feet away from us.  I felt like he was at pretty close to our altitude, but again, as the linked question says, that's hard to say for sure.
The linked question points out that distances and altitude differences are hard to judge.  That's fair, so I'll offer the following bona fides:

I live about half a mile from a commercial airport, perpendicular to the end of the runway, so I have a good idea of what jets that are half a mile away from me look like.  I would estimate that the other jet was a bit but not a lot further away than that.
If two jets pass by each other at 1/2 mile, and you can see another jet that is 60 degrees in front of or behind you, and he, like us, was travelling 480 mph, he would have been in my field of vision for 6.5 seconds.  That sounds about like what I remember.  At 3 miles, that number turns into 40 seconds.  I don't remember the exact amount of time, but my initial estimate before I did this math was 8 seconds.

I could imagine our pilot banking the plane simply to lessen the odds of the passengers noticing a safe encounter, as even a safe encounter leads to awkward questions on sites like this.
So, my question: Did we have a near miss, or was this a standard close-but-intended encounter?

The answer to this very similar question is a good one:  https://aviation.stackexchange.com/questions/3680/how-is-a-near-miss-defined

Thanks for posting the # and specifics, often these questions omit those crucial details. Did you happen to see any details of the other plane? Airline logo, etc.? Was there any weather in the vicinity, such as thunderstorms?

Clear blue skies, minimal turbulence.  I couldn't see what airline the other plane was.

Possible duplicate of How close can planes fly to each other over the ocean?

What makes you think the turn had anything to do with the other aircraft at all? You were probably just turning to follow your route. Planes turn multiple times during a flight. And sorry, but the fact that you live close to an airport does not mean you are qualified to determine the vertical distance between two aircraft in the air, which would be especially difficult during a turn since your frame of reference would be tilted.

The basic rule of aircraft (and ship) collision avoidance is "if the thing you are looking at is moving across your field of vision, you aren't going to hit it". So if you see something 45 degrees way from your plane's course and travelling parallel to your plane in the opposite direction (and therefore the 45 degree angle will be increasing), that is nothing to worry about.

From the flightradar24 playback, it appears to have been an encounter with Etihad Airways 171.
Your flight (Alaska) was at FL360, while the opposing was at FL370. Therefore, there was 1000 feet of vertical separation between them, which is typically considered safe when operating with RVSM, Reduced Vertical Separation Minima rules (see other answers).

I also looked up the LiveATC recordings archive (Seattle Centre Sector 14 high, 28/04 2000Z), and found a clip that I think corresponds to the event:

Alaska 557, can we take a heading right of course to avoid some wind?
Affirmative
Ok, deviation approved [...]

The other answer is good because it applies generally, but I like this one a lot; it directly reassures OP that everything was safe.

Also, to a casual observer, other aircraft often appear to be much closer than they actually are.  People are used to cars and fail to appreciate how the lack of visual references and the size contrast play with their perceptions.

Thank you!  The fact that it was a 777 rather than the 737s I usually see locally means that it was about twice as far away as my initial guess.  It looks like the closest approach was about 2 nm.  In the end, it was a random course change that happened to correspond to a normal close encounter, not some attempt to hide the other plane from us.

FLNaN undefined passing near Hackamore makes one wonder... :)

I actually wonder now if the "wind" on the radio is wake turbulence from the 777. Is 1000 feet a reasonable separation to avoid that? It wouldn't have to be dangerous necessarily, just a pilot wanting to keep his passengers comfortable.

It's also worth noting that about 5 minutes after you passed the Etihad airliner, you passed KLM 183 (a 787 Dreamliner) with the same vertical separation and perhaps even closer horizontal separation.  But you didn't notice it because it approached from the right side of the plane.

@AricTenEyck two nanometers is awfully close for a pair of tin cans moving at 1000 km/h relative to each other...

@JohnDvorak pilots in both planes are good at flying close to each other. That’s whya big pay check.

@FreeMan: What you're thinking is probably correct. It's an alien spacecraft and they use JavaScript to run it.

Geez... Now I just noticed that Hackamore is just northeast of Scarface. Scary place! FL370 is about as close as I'd want to be!

@JohnDvorak: https://www.youtube.com/watch?v=S0-32x4hD8o

@JohnDvorak 2nm = two nautical miles

@customcables067 that would be nmi. nm are nanometers.

probably so, but the slang in the aviation community is to interchange nm for nautical miles. You are probably technically correct, though.

It's unlikely.  Because of RVSM, Reduced Vertical Separation Minima rules, the vertical distance between airplanes passing each other is only 1000 ft.  If you were in the flight deck so you could see out front, you'd be having a wonderful time freaking out because airplanes on the same airway pass above and below going the other direction only 1000 ft away. And when they pass over 1000 ft away it looks like 300 the first few times you experience it.
Plus, thanks to GPS, everybody is exactly on the center of the airway all the time, so every oncoming airplane seems to be coming straight at you until they are a couple miles away and when they go by they are perfectly lined up above or below (the passengers rarely see them all as a result and have no idea that they are regularly are passing other aircraft close by like cars on a highway).
Same with airplanes crossing your track, or going the opposite way on an offset track (those are the ones that passengers see).  If they happen to pass directly above or below, or close by going the other way, same deal - only 1000 ft above or below if they are on the next flight level and it looks like they are right next to you.
The other thing is that collision avoidance systems (TCAS) give a climb or descend instruction when there is a need to take evasive action, not a turn, so if there was an actual TCAS event you'd be pressed into your seat or coming out of it, not turning.
That being said, it is possible that one of the crew saw something visually and decided to maneuver laterally to avoid it (it was likely someone hand flying because autopilot turns are pretty gentle), but keeping in mind that fact that airplanes normally pass really close vertically, and serious evasive action usually results in stuff flying about inside, it's a good chance that what you saw was normal.

So why the sudden deviation?  It's clearly visible on the FlightAware ground track.

Who knows?  They could have been avoiding someone, or ATC could have asked for a fast heading change to avoid a conflict.  My point was just not to make assumptions because you see other airplanes nearby.  If there was something serious there is likely an incident report.  I know of a Dash 8 that encountered some sort of unmanned drone at 15 or 20000 ft and pushed over to avoid it, injuring the FA.

@AricTenEyck Planes do tend to turn every once in a while. It makes it easier to hit the right airport. It has nothing to do with other planes that are vertically separated.

FYI, auto of autopilot in RVSM space is mandatory--no hand flying.

If you're in RVSM and you get a TCAS RA, or you see something you need to avoid right now, you are disconnecting and hand flying post haste, RVSM or not.

@AricTenEyck Queue Monty Python skit: "Captain, I see on the manifest that avid Aviation StackExchange user AricTenEyck is in seat 10".  "Roger that, executing hard right turn, that'll give him something to talk about!"  "Just as that 777 is coming up on our left! Teeheeheeheehee!" "NEAR MISS!!" Teeheeheehee"  "Oh, you forgot to turn on the cabin mic..."  "Oh...  well maybe next time!"

If the aircraft was flying above, it might have been to avoid crossing its wake turbulence.

The Etihad flight was a 777, which is a much larger aircraft, but of almost the same proportions as the 737.  Also living near an airport, I am well familiar with how difficult it is to gauge altitude.  You see airplanes apparently moving at quite different speeds; actually a "slow" plane is moving at a similar speed but is larger and farther away.  It's really a problem in open air, with no reliable visual references for size.
So I suspect you saw the same optical illusion, where the 777 seemed closer simply because of its large size.  That said, it was quite close vertically - 1000 feet separation. The horizontal separation did not matter because the vertical separation was sufficient.
It's quite possible the wake turbulence from that huge 777 was of concern to the Alaska pilot, so he deviated (with permission) to avoid it.


text:
What do these numbers above the terminal jetway mean for pilots?
The sign above the terminal has numbers, I assume, to help the pilot park.  What do they all mean?

To add to ymb1's answer about the -67, the other information on the sign indicates whether the flight will be arriving or departing (in or out), the flight number (2110), departure/destination airport (PSC), and scheduled time (10:00).

Shouldn't your answer be actually "merged" into ymb1's answer? To make it a good quality single-answer solution? Instead of turning this site into discussion forum.

This is at Sea-Tac Airport in Seattle. The -67 indicates that the flight is supposed to depart in 67 minutes, and gives flight info like flight number, destination, and updated departure time. This is used more for the ground staff. Everything works on a timeline. At Horizon air, where this is pictured, the process usually begins at 40 minutes prior to departure (minimum turnaround time for us). Boarding begins at -30, and ends at -12. If all goes well, the brakes are released by the time the counter reaches 0, which determines the on-time status of the flight (at least for the company, now it's up to ATC to get the flight out). Only the C gates at SEA have the display.


text:
What is the time information on the G1000 MFD map during a hold?
I just completed an ILS approach and went missed.  I followed the published missed approach and entered the hold.  When I entered the hold, on the MFD map a timer-like display was on top of the "aircraft" e.g. 00:00
The timer display did not increase after being in the hold for about 5-10 minutes.  I even started the timer from the TMR/REF softkey on the PFD to see if it would change the display on the MFD, and it still remained at 00:00.  Does anyone know what this timer display on the MFD during a hold is?

Is it possible that the fuel totalizer was not reset, and the G1000 thought your aircraft was out of fuel? That sounds like the range ring behavior.

A timer on the MFD would probably be for your Top of Descent - could be due to several previous actions on what the FPL contained before and during your hold.
PS - G1000 as primary means of navigation for an ILS hold? :D

It's most likely an Expect Further Clearance (EFC) timer. You didn't mention the aircraft type but there are several references to it online: Garmin service advisory; Seneca V impressions; Archer manual quoted in a forum. My G1000 experience is in a C172 and I've never seen it there (I can't find it in the manual either) but the features are different depending on the aircraft, so it's possible that EFC timing just isn't in the C172 version.

Related: Expect Further Clearance


text:
May planes be flown above the maximum certified altitude?
Let's take the 737NG as an example. The 737NG has a maximum certified altitude (MCA) of FL410.
However, looking at the MCP, the maximum altitude that can be set is 50,000ft. And the pressurization panel can be set to 42,000ft.  All allowing to fly above FL410.
Please note that I am getting this information from FSX with the PMDG 737. Though a simulator product, it claims to be very accurate model of the plane.
Also, although I am not sure, higher altitudes = fuel savings right?
So this rose the question: Can/May I fly above the maximum certified altitude?
Can I fly safely with an 737NG at 45,000ft for example?
Related: What determines the maximum altitude a plane can reach?

https://www.flyingmag.com/whats-41000-feet
And also
http://www.nytimes.com/2005/06/14/us/just-before-dying-a-thrill-at-41000-feet.html

Related

FL410 is not always equal to 41,000 feet due to weather. You could be be flying at 41,000 feet but at FL390.

The Max Certified Altitude is the highest that the manufacturer has test-flown the aircraft to. (And the 100ft simply makes cruising at FL410 legal.)
If you want to take the airplane above the MCA, and you can actually get it up there, then you now become a Test Pilot, but without the Test Pilot pay and insurance cover. Good luck!

I’d highly doubt that Boeing never flew the 737 NG above 410; that altitude is as much based on time to descend for a rapid depressurization as on anything else. Light enough, 420 would probably be easy (tho above 410, 430 is the next discreet, assignable FL, and you’d need to be quite light in a 700 for that to be comfortable). That said, your main point is correct: can you do it, maybe. Can you do it legally, no. (Emergency authority - TCAS RA - excepted.)


text:
Do planes use autopilot when flying in formation?
I've attempted to fly formation over a multiplayer flight simulator, and it was incredibly hard. It felt really hard to have the two planes go exactly the same heading, so it was always necessary to sway the yoke back and forth, constantly overshooting and undershooting.
When real planes fly formation, do they use some sort of short-range radar system and an autopilot to maintain the formation shape? I could imagine that manually flying formation would cause extreme fatigue due to the need to constantly fiddle with trim and yoke, and would be very dangerous.
Or is this simply a case of actual pilots having actual training, instead of my amateur "training" of fiddling around in the simulator overusing instruments?

Nope, pilots do not use autopilot to fly in formation. An autopilot cannot achieve the precision required for flying formation (e.g. keep the wingtip aligned to that door). Usage of autopilot would also hinder responses when an immediate correction is necessary.
You may refer to the TV episode Jet Stream and Blue Angle training videos on the internet. From the pilot's perspective, the planes are always moving around, though in a confined space. Continuous small adjustment is necessary. From the ground, the small floating cannot be noticed visually because the observer is too far away.

P.S. Based on my 17 years of experience with PC flight simulator, flying in formation in multiplayer is definitely doable, and can become second nature with practice.

Mostly correct, some components of the autopilot in aircraft like the super hornet are normally engaged during flight, like autotrim.  But you are correct, small constant control inputs are needed throughout the formation, most notably in the throttle.

"Usage of autopilot would also hinder responses when an immediate correction is necessary."
Wouldn't autopilots, if designed for this purpose, typically have a far better reaction time?

@user54609 the problem is autopilots are never designed for this purpose. They are designed to fly based on instrument readings, not based on the relative position of another aircraft. An autopilot also does not break off if a danger develops.

@user54609 Correct, computers can be made so that they can fly formation better than any human can, but they must be designed to do so and have a frame of reference to the other plane other than visual references. It is still too unreliable for computers to discern deviations just on sight. If they would create electronic means to detect attitude change and relative position to the lead aircraft (and cross-talk between aircraft computers), then yes.

Computers can be programmed to fly formation, but there are no autopilots in production yet that can handle it. The industry is trying to develop it so that airliners can fly in a V-formation in the cruise, like birds, in order to save about 5% in fuel. It would also alleviate congested skies if airliners could pair up. This would need very accurate positioning sensors that also work at night and in 0 visibility.
The secret is to make small control adjustments. One will be oscillating with large amplitude, but low frequency. Learn to control it (straight-and-level first of course) and slowly bring it closer to the lead a/c. Your brain will recognize patterns of how to react to disturbances over time.

Comments aimed at other answers really should just be comments on those other answers.  The second paragraph though, is some very interesting information.  Do you have any sources to back that up?

http://news.stanford.edu/news/2009/june3/birds-060309.html

Although I can't find a reference to where it said that they were actually working on it.

As far as fighter aircraft or small light aircraft flying as the wingman in formation, I agree with that's been said in the other answers.  However, larger aircraft than these can fly formation, and in some cases autopilot use is a little more common than these answers might lead you to believe.
First, the lead aircraft has an entirely different task than the wingmen: his objective is to be a stable platform for them to maintain formation off of, and so for Lead to have the autopilot engaged is entirely possible in many scenarios.  (Not so much if Lead needs to do lots of maneuvering, but if you're going a long distance straight & level, the autopilot can do that very precisely, leaving the pilot(s) free to devote attention to whatever other tasks may be required: communicating with AWACS/wingmen/Command Post/ground party/other flights, assessing the situation with regards to threats/weather/mission/whatever, eating lunch, etc.)
In the case of Aerial Refueling, it's fairly common for the tanker to be on autopilot, since this gives the most stable platform for receivers.  When the receivers are more nimble than the tanker, this isn't such a big deal; when the receiver is another heavy, it helps.  When the receiver is really heavy and performance limited, having a stable lead (i.e. the tanker) helps a LOT.
Beyond this, wingmen can also be on autopilot if the type of formation allows it.  If you're stacked 500' above and a mile behind Lead, crossing the ocean for the next several hours, there's no reason at all not to use the autopilot!  If you have the waypoints that Lead is using, you could even couple the autopilot to the FMC, and just adjust the throttles to maintain the desired spacing.  If not, then you manipulate the heading bug to keep the position you want left/right.  At the point when you get ready to do something like airdrop or a formation landing, the autopilot will come off, but for a long, straight cruise portion, it's entirely usable.


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What are the most number of propellors that a rotorcraft has been flown with?
If you had 1000 small propellors you could accelerate upwards many times faster than with 4 large propellors.
What are the rotorcraft experiments with the most number of propellors ever?

"If you had 1000 small propellors you could accelerate upwards many times faster than with 4 large propellors." Could you, though?

As @egid said, this number would need to be supported by a reference. The answer is probably in the Guinness book, but you may search in the direction of hover-board. However this is going to vary with payload weight, distance flown, altitude, etc. And nothing forbids to stick two boards together to double the record.

In a related matter, i was searching for references regarding walnut breaking competitions, guinness is a serious reference clearly: https://www.youtube.com/watch?v=S4BuThlqA7I

"If you had 1000 small propellors you could accelerate upwards many times faster than with 4 large propellors." Not with the same total disk area and installed power. One big rotor is always the best.

This guy is actually flying with 76 rotors.
But I might argue that your "more rotors is faster" idea may be flawed, smaller propellers are less efficient (because the further from the center you are, the faster the blade is moving, if I'm not mistaken), so the trade may be fewer, more efficient, propellers for a larger number of less efficient ones.

your point is correct in part, however a long blade with a faster tip will get close to the speed of sound more quickly, so you probably have a limit here...

As far as I have come across there was a guy who had mentioned of working with 36 motors on a multirotor.


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What is the correct name for this ILS Indicator?
What is the correct terminology for the name of the aircraft instrument that looks like this?

Good writeup here: https://aviation.stackexchange.com/questions/26031/what-are-the-differences-between-an-obs-a-cdi-a-heading-indicator-and-an-hsi

That is the course deviation indicator, or CDI.

Smashing, a quick Google and all the information on it I could ever want!

That image is a basic representation of a course deviation indicator, as mentioned by Steve H. An actual gauge would look like this:

Image Source
In your image, it shows that the aircraft needs to correct to the left and a little upwards, so that it will be on the glideslope as shown here.

Am I misreading it?  It seems to indicate a little right and high, so correction is left and down? Mind you, I don't have an IR :)

The crosshair is the location you want to be in; the circle in the centre is the aircraft. So, in the image in the question, the aircraft needs to move up and left to be on the crosshair.

@Simon I'm not instrument rated either. According to my understanding, the circle is the aircraft, horizontal line is the glideslope (see the 2nd image in my answer), vertical line is localizer. You need to be left and up.

@Farhan that's correct assuming you're not flying the localizer back-course in which case it would be reverse sensing. You wouldn't generally be using the glide slope in that case though. There have been incidents attributed to unfamiliarity with instruments such as the artificial horizon, iirc russian aircraft traditionally showed the aircraft attitude rather than the position of the horizon (i.e. the exact opposite) and there might well be some old CDIs around showing the opposite of what you would expect as well.

@anaximander Maybe I need some training.  If the circle is the aircraft, is it not showing that it's low and left?

I am not allowed to comment here so I have to make an answer. Steer towards the needles! Use this memory rule and you can figure out where the GS and the plane is.

I think part of the problem here is that there are two CDIs above.  The top one shows the plane low and right, the second low and left.. although the second one is nearly centered.

@falstro - you should set the published front course when flying the LOC B/C. It should not be reverse sensing in this instance. You are correct in that you should not fly the glideslope in this instance, however

@SSummer an HSI would indeed not be, but a standard old school cdi doesn't care which course you set and will be reverse sensing on the b/c.

authors: Στέργιος Μέκρας


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Can bird strikes be dealt with by throttling down engines to idle?
From a related question, I got to thinking about bird strikes again and the fact that they are sucked into the engine at great velocity.
If a pilot had 5 to 10 seconds of warning time, would throttling down the engines to idle be a good idea? Would it save those engines even if a bird hit the intake fan? This would of course be a temporary measure, intended to prevent the birds being sucked in harder or faster than otherwise. Once the birds passed, the engines would be throttled up again.
I'm thinking about large commercial jets like the Airbus A320.
EDIT: one concern I have is that at high throttle, the engine will suck in objects from a wider area than just the engine inlet. Is this assumption true?

Another thing is that an engine at high power will take more severe fan damage from ingested foreign matter, but is more likely to centrifuge that foreign matter into the bypass duct instead of ingesting it into the core, where it can really break stuff!  (This was discovered during the TACA 110 investigation.)

A goose can easily be flying at 40mph and a commercial jet will be landing at about 150mph, giving a closing speed of about 190mph (305km/h). That means that, to spot a goose with 10s warning, you would need to spot it when it's more than half a mile away.
Not only would you have to spot that tiny speck of a goose half a mile away during the busiest phase of the whole flight, but you'd also have to immediately work out whether the tiny speck was even coming towards you. If it was, you'd have to estimate whether it would even hit the plane.
Then, supposing you'd immediately and accurately recognized the danger to your plane from that tiny speck, you'd have to immediately reduce power to the engines, while being very careful not to hit the ground the bad way.
In short, regardless of how much better the outcome of a bird hitting an idling engine would be than one hitting an engine under power, it's just not feasible to try to power down the engines. Once you take into account the false alarms, which would vastly outnumber the actual bird strikes, trying to power down the engines would probably be far more dangerous than just hitting the bird.

It should probably also be noted that final approach is not a great time to be fiddling with engine power. Better to just go around if you see something that might compromise the landing.

In general, greater the engine thrust, greater the damage caused due to the bird strike. This is to both the higher speed of the compressor blades and the pressure ratio.
So, it makes sense if the pilot can throttle down the engines if bird strike is imminent. In fact, approaching in idle condition is on of the strategies for reducing the possibility of bird strikes. See http://www.boeing.com/commercial/aeromagazine/articles/2011_q3/4/ and http://www.skybrary.aero/index.php/Bird_Strike_on_Final_Approach:_Guidance_for_Flight_Crews.
However, there are other things to consider. For example, most of the bird strikes happen during take off and landing and it is not advisable to throttle back the engines at that point of time (it is one thing to descend in idle power and another to reduce power during landing).
Also, given the speed of the aircraft and the size of the birds, it is unlikely that the pilot will have enough warning unless he's approaching a huge flock.
Another thing is, given that the velocity of bird strike is a function of both aircraft speed and compressor speed, reducing to idle power won't prevent damage due to bird strike as the compressor rotates at a very high speed even in idle condition.
In any case, modern engines are designed to operate (or fail safely) in the event of bird ingestion and it is far better to continue the flight and take appropriate action like go around and land.

"would throttling down the engines to idle be a good idea?"
Probably not, 3 reasons I can think of.
Landing speeds for an Airbus A320 (your example) will typically be in the region of 130kt to 140kt faster then the cruise speed of many GA aircraft. Even at those speeds, a bird strike is likely to have a significant impact on anything it hits.
The published minimum speed of the high pressure compressor/turbine spool is 66% of the rated maximum. Since the max turbine speed is VERY fast think 20-30k RPM, 66% of it is probably enough to cause any damage that would occur with a normal strike.
In the unlikely even you see the birds with more than 2-3s to react (there very hard to spot at high speeds), you would more likely want to INCREASE thrust to A) try to outmaneuver the bird and B) in case of an engine failure the other would already be running near the power required for continued flight on one engine (essential since most bird strikes are at lower altitudes where you may not have enough altitude to reduce power for 10s or so then power back up to max thrust taking another few seconds)
EDIT: an argument could be made for not touching the throttles at all until you can total verify one or more engines have failed based on the number of pilots, who in a panic manage to turn off the wrong engine during an emergency.
Note. referenced speeds are approximate I would welcome edits with more specific data.

Out manoeuvre a bird?  I think not.

The question talks about 5-10 seconds. That is barely enough for the engines to spool down. The speed won't change significantly in that time and does not seem to be the point.

I couldn’t find any quotes on how long it takes from cruse to flight idle, but it is a moot point as flight idle is still a few thousand RPM. as to out manoeuvre a bird. "5 to 10 seconds of warning time" [improbable I know] might be enough to clime an extra 40 feet.

Since the pilot is not expecting to see a flock of geese ahead of him, there will be some delay in his response as his brain processes this unfamiliar object(s) in front of him, and formulates a plan to deviate/slow down.  This reaction time could easily be 5 seconds.  Five to ten seconds warning time is therefore 0 to five seconds, clearly not enough time for an effective aircraft diversion.

To answer your question "Can they be dealt with", I think throttling down is nowhere near a total solution, although it might have a small effect.
The big problem is that the aircraft is typically flying at a speed of between 130 and 250 knots towards the birds. The engines don't suck the birds in (because there is relatively little time for this to have an effect) the birds just get in the way of the engines and the impact forces of a 10 lb bird doing between 130 and 250 knots (relatively) do the damage.


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What is "Bird Strike Speed"?
I read through the reference speeds of the Hawker 850XP and I stumbled upon the "Bird Strike Speed". It is specified as "280 KIAS up to 8000 ft".

What does that speed mean?

LOL. Maybe its like turbulence penetration speed, its the best speed at which to penetrate a flock of birds? Apparently the speed is different in cold weather.

@Devil07  Yes, in cold weather you have to worry about hitting frozen chickens -- they're harder on the windshields, as the Mythbusters finally figured out (on the 3rd try!).

Aircraft structures are required to withstand the structural damage caused by birdstrike. According to 14 CFR 25.571 - Damage - tolerance and fatigue evaluation of structure:

(e)Damage-tolerance (discrete source) evaluation. The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of -
(1) Impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane's flight path is equal to Vc at sea level or 0.85Vc at 8,000 feet, whichever is more critical;

In case of aircraft with high cruise speed, the speed may be limited below 8000 ft to comply with this regulation. As Bird Strike Requirements for Transport Category Airplanes notes,

... to meet current bird strike criteria, some manufacturers specify relatively low $V_{MO}$ and $V_{C}$ airspeeds up to 8000 feet, that increase above that altitude.

The 'bird strike speed' is likely this.

What about a 20 pound Canada Goose?

@Devil07. Assuming the limit is a kinetic energy limit, KE=1/2 m v^2, so if m goes up from 4 to 20 pounds, v^2 needs to reduce by 5, hence I say 125 KIAS for your goose (unfrozen).


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What do pilots do on a windshear alert after V1?
What is a pilot taught to do, when a windshear is detected and the planes speed is above V1 and before Vr? I can't think of what is safer.

Rejecting, with the risk of overshooting the runway.
Taking off, with the risk of the windshear stalling the plane and dropping it on the ground. If the height of the aircraft was low it probably would just overshoot the runway. If the aircraft already gained some altitude, it could crash.

As well, are there any differences in procedures if the alert occured

After V1 but before Vr
After Vr

After Vr you would have already taken off. No chance of rejecting at that moment.

What do you think V1 means?

V1 is the calculated decision point at which takeoff must continue. Unless the aircraft is suffering a catastrophic failure, the pilot should not abort takeoff after V1.
The danger when taking off during windshear is the sudden lost of airspeed may result in the airplane settling back on the ground after it has become airborne. The technique, therefore, is aimed at gaining ground clearance in the shortest amount of time possible. In general, this involves:

Apply max thrust
Delay rotation
Once rotation is initiated, quickly pitch up to gain altitude

Below is an extract from the Boeing 777 FCOM about taking off when windshear is suspected (my emphasis):

Takeoff with less than full rated thrust is not recommended (...)
Use the longest suitable runway (...)
Consider increasing Vr speed to the performance limited gross weight rotation speed. (...) If windshear is encountered at or beyond
  the actual gross weight Vr, do not attempt to accelerate to the
  increased Vr, but rotate without hesitation.
(...) Minimize reductions from the initial climb pitch attitude until terrain and obstruction clearance is assured, unless stick
  shaker activates.
(...) Stick shaker must be respected at all times.

What pilots are taught is that V1 is the go/no-go decision point.  So after V1 they will continue with the take off, but the PIC is the final authority as to the operation of the flight and may take an action that he or she determines is best depending on the specific scenario.
Regarding Vr: pilots are trained to do thorough pre-flight planning and become familiar with the flight plan and weather conditions at the point of departure, route of flight, and destination at the calculated time of arrival.  The pilots should know if there are windy weather conditions, an approaching front, or storm so that they can delay or cancel the flight or increase the Vr speed for takeoff from the standard Vr speed and have some extra buffer.  That way if there is windshear they have a higher safety margin between their climb speed and stall speed.

In case of an engine failure, the total available thrust to the aircraft be reduced by 50% and it will lose at least 80% of the available climb thrust.  Suffer wind shear in these conditions, the airplane may not be able to sustain a positive rate of climb on one engine, making it very dangerous at low levels.  If low level wind shear conditions are anticipated at or in the vicinity of the airport, the pilot should consider this as part of the pre-takeoff briefing or postpone flight operations until conditions subside.  Have a plan based upon the procedures listed in the AFM and stick with it.  If an LLWAS alert is heard or if wind shear is encountered past V1, in general you’re past the point of no return and the airplane is going flying - unless you’re absolutely certain you can still stop it with the remaining runway available at your current takeoff weight, but that’s up to the captain to make that call.  Most jets have a considerably higher OEI performance margin than do light or medium twins and would take a pretty strong wind shear in order to threaten climbout on OEI.  Some things a pilot might do in those circumstances.

Advance both thrust levers to maximum.
Delay rotation or remain in ground effect until V2 has been attained.  Accept additional touchdown on runway of airplane cannot sustain a climb until V2 is reached.
Select a Vr with an appropriate wind shear margin added on.
If sustained positive rate of climb is possible and attained, retract the gear to remove parasite drag.

retracting the gear is exactelly the wrong move: think about it, if you are climbing steadly, then why get greedy and retract the gear? if you are not in a sustained climb, then why retract the gear at all? both cases it’s a wrong move, guaranteed to fail your checkride

Because you will get rid of the parasite drag faster and accelerate quicker to V2, allow for a greater rate of climb once you are maintaining V2 or flying at Vyse.  And unless you have a manufacturer recommended OEI procedure saying otherwise, you will retract the landing gear once a positive rate is attained. Any examiner who would fail you for that is a moron who has no clue about flying multi engine aircraft.

seriously: retracting gear(or flaps)  in windshear is a ‘fail-able offence’ :-)  where I live in EASA-land .I understand it seems counterintuitive,  I just always supposed  that the action of ‘retracting’ the gear will create more drag initially with doors opening etc. I’ll look for the actual regs on this one tomorrow morning

No, the name of the game here is to get clear of the runway and obstacles as quickly as possible.  Once you hit V1 1) you’re going flying at that point, so when Vr comes you will establish a pitch up appropriate to the aircraft type and depart.  If wind shear is encountered at that point a pilot will ensure a positive and sustained rate of climb is indicated, then retract the gear.  The last thing you need at that point is the gear hanging out of the airplane and impeding climb performance.  You don’t need to divert extra climb thrust to fighting through the parasite drag of the gear.

@Radu094 Acceptable Means of Compliance and Guidance Material to Part-FCL, AMC1 FCL.930.MCCI, Part 3 point (b)(7), looks relevant, though not authoritative for what you're after.

These guys do a great example of departure procedures in pre and post V1 wind shear in an A320 simulator.  Note the FO retracts the landing gear only after a positive rate of climb in obtained and keeps the gear retracted throughout the emergency.   https://m.youtube.com/watch?v=nCy1tAfLbDg

@Carlo in the video the gear is retracted 1 sec BEFORE the windshear aler, probably a concesion from the examiner. Please see the procedure card top-right at the 0:00 : do no change configuration (slats/flaps, gear) until out of windshear. I’m not gonna get too religious on this topic, so I am  not gonna say more than this:: retracting gear is against current operational practices

And I’d answer back that you do see evidence of wind shear even before the EICAS alert for it.  Look at the airspeed trend during the takeoff roll and departure.  The acceleration rate is not uniform - a telltale sign of wind shear in the area.  Once airborne, the situation allows for gear retraction so the crew opts for it.  As such the crew responds correctly- note they follow the wind shear checklist and don’t change the configuration once the EICAS alert is received.  The pilot handled the situation correctly and any examiner who would fail him for that is a fool, pure and simple.

What I would do is to keep full throttle, and try to hold the plane in ground effect until reaching a fairly high speed. Then, start increasing altitude slowly and carefully...

But the question isn't what you would do; the question is what pilots are taught to do.

The question is (poorly) states both ways.  Look at the title.

If windshear occurs when the plane is slowly gaining altitude, the plane may drop back onto the runway hard, leading to structural damage and potentially loss of control.

You're going to fly right into the hill/building at the end of the runway.

Windshear alert will not occur while on the ground on Airbuses; you will only get the alert once airborne, after Vr. I suspect most manufacturers have implemented a similar feature, perhaps, in order to prevent pilots from rejecting after V1.
What to do is usually a precise and exact procedure. While it does vary from type to type the main items are in common:

Use full available power
pitch up for a max rate of climb
maintain a/c configuration (no gear or flaps retract)


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What uses for flight did early aviation pioneers envision?
Between 1903 and 1914, what did the aviation pioneers think flight would be used for, besides military purposes? Did they envision anything like crop dusting, commercial transportation, charting terrain in the wilderness, scientific study of the atmosphere, etc.? Even just pure entertainment flights?
I did read this little article about the Wright Bros thinking it would actually deter war, but it didn't mention if they had thought of some alternative use to warfare.
There are of course many aviation pioneers from those days. Did any of them envision uses besides war? It would be really interesting to see what they came up with and if it turned out that way.

@Squareoot Not interested in any one specific country. I'm interested in the early aviation pioneers of any and all countries.

Burglary was one of them: http://paleofuture.com/blog/2009/12/8/burglars-of-the-future-1910.html
http://paleofuture.com/blog/tag/airplanes has some interesting stuff on early flight, though it does run past 1914

I'd like to point out that at least one of the aviation pioneers (Santos Dumont) allegedly hanged himself due to the use of aircraft in warfare.

"Airship" originally meant any large aircraft, whether lighter or heavier than air. Ballooning was already a popular sport among the rich when aeroplanes first flew. Most pioneers seem to have taken it for granted that aircraft - both airships and planes - would be used for private and commercial travel as well as warfare. Airmail was perhaps the most discussed niche role. A few might have had other niche purposes in mind but on the whole they were content just to get the thing flying and find a buyer or sponsor. A V Roe built the first plane with an enclosed passenger cabin in 1912, the first year a mail package was delivered by air.
J W Dunne was one of the few influential pioneer aviators from a military background and argued for a variety of specialist types for different military roles. He was also a friend of writer H G Wells. Together they envisaged total war with vast airship fleets capable of wrecking whole civilizations and hoped that the threat of this would put an end to ground warfare as such. When Wells wrote "The War in the Air" (around 1908?), Dunne sent him some of his research materials which found their way into the story.
Rudyard Kipling (of Jungle Book fame), though not an aviator himself, wrote a novel about a commercial transatlantic mail airship, "With the Night Mail" in 1905.


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Can pop-out floats be installed post-manufacture on all helicopters?
I have seen in many helicopters pop-out floats installed.
Are they the optional, or do they come pre-installed with every helicopter?

Pop out floats can be installed easily on any helicopter with Tubular landing skid type landing gear. Its essentially an auto-inflatable tube attached to the landing gear. Have a look at this:

How do they get inflated when in contact with water? Do pilot have an option for this?

Yes. For obvious reasons, or else they'd get inflated on a land touchdown as well

@NitinG: Might be either, depends on type. (Obviously sensor that detects being immersed in water won't trigger on land)

in the copter I've flown with pop-out floats (R44), there is a nitrogen bottle under the left seat, with a cable-operated valve that goes to a collective-mounted handle that can be operated without taking your hand off the collective. the handle has a safety on it, which is disabled "feet wet" and enabled "feet dry". here's a photo: http://www.helicopter-redeye.com/images/F%20View%20from%20above.jpg

downvoted b/c popout floats CANNOT be installed on any copter. they can only go on copters for which an STC has been developed

I was told by an avid flier that you get "Customized" popout floats, which need to be bolted (tied) in order to be deployed. No STC needed.


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Why is managing CG (centre of gravity) important?
Why do people have to worry about CG? Is it primarily due to fuel penalty, or take-off and landing performance, or is it mostly just to keep the aircraft from tipping over during loading and unloading, or other?
As a bonus, does tipping over of aircraft actually occur these days due to improperly managed CG?

a badly managed CG can crash your plane

It's all about stability in flight (and, more importantly, ability to recover from a spin should one be permitted to develop). None of the factors addressed in the question are a CG concern that I've ever heard.

Why worry? Because you want to live. Does tipping (assuming you mean loss of control) happen because of unmanaged CG? Yes.

Why do people have to worry about CG?

Because the manufacturer has designed the aircraft following certain criteria, and one of those is where the CG will be during flight. If the CG is outside that interval, the elevator will not have enough autority to keep level flight.

does tipping over of aircraft actually occur these days due to improperly managed CG?

Was not improperly "managed", but it went out of bounds (in particular, the aft limit), unfortunately, soon after take-off and a 747 with 7 people on board was lost.

wow. good example. For some reason I was not thinking of the obvious when I raised the question. I had just read an article about CG as it relates to a bunch of secondary issues. Thanks.

In particular, as mentioned before, an out-of-limits CG may make a stall recovery difficult or impossible (outside of the other handling problems the CG may cause).

Video is of National Air Cargo, 747-400 freighter, N949CA, Bagram Apr 29th 2013

@RedGrittyBrick, what is the meaning of "gear up" in that video? Is that mean to pull up the landing gears?

To balance the aircraft, the sum of the lift from all lifting surfaces has to be at the same position as the weight. If the c.g. is ahead of the center of lift, the aircraft will pitch down. The pilot can counteract this by pulling on the elevator, which in effect reduces lift at the tail and thus shifts the center of lift forward. As you can imagine, there is only so much lengthwise change which can be effected with the elevator. So the first limit for the c.g. position is given by controllability.
Another important consideration is stability. You want the airplane to return to the original flight attitude if it has been disturbed, say by jolt on the stick or yoke. This can be done by creating proportionally more lift with the forward surfaces (= the wing in a conventional configuration) than with the tail surfaces. Say the jolt pitches the aircraft up. Now the aerodynamic forces should change in a way which increases lift proportionally more on the tail surfaces, so the aircraft pitches down and returns to it's original attitude. By distributing lift the way I mentioned above, the same change in angle of attack on both wing and tail will create a proportionally higher lift increase on the tail, making the aircraft (statically) stable. Thus, the second important limit for c.g. location is stability.
As for tipping over …

good explanation and pictures

http://www.ultraligero.net/Cursos/mecanica/fundamentos_de_la_mecanica_de_vuelo.pdf Chap.9
Effect of cg position on angle of attack stability
could you please point me out where I am wrong?

@Federico: Yes: The neutral point is fixed and is the point where the air loads can be expressed as a sum of a variable force plus a constant moment. This is the quarter chord point of a single wing and is used to express the c.g. position (26% of MAC would mean the c.g. is 26% of the mean aerodynamic chord ahead of the neutral point). The aerodynamic center is the point where all air loads can be summed up without leaving a residual moment. This point is variable and moves forward with higher AoA and can be shifted with the elevator.

@Federico: Wow - I just read the PDF you linked, and the author has them confused, too (chapter 1.6 versus 8.3). He uses the same term "aerodynamic center" for both concepts. He also uses the term neutral point, but only to express the stability consequences when the c.g. is at this point. I need to read more, but from a first glance I would say he has not really understood the difference. Maybe this is a good topic for a new question here!

@PeterKämpf, what is the center of lift here? Is it the wing?

@AirCraftLover: The center of lift is the theoretical point in which you can replace all vertical pressure differences (i.e. lift) over all surfaces with a single vector without producing a pitching moment. It is also called the aerodynamic center.

In flight the wing is generating upward lift, the tail plane a downward force. As you move the c.g. forward, more downward force is required from the tail plane, which means the tail plane is causing more drag, which means you will burn more fuel. Move the c.g. aft and you'll burn less fuel. However, the farther aft you move the c.g., the less stable the airplane becomes in response to gusts. As an example, 747 freighter operators typically aim for a zero fuel weight c.g. of 26% MAC (mean aerodynamic chord) as an optimal balance.

good point on the fuel